"I looked like an aerobics teacher," the 27-year-old actress, who famously shed weight for her Golden Globe-nominated role in The Devil Wears Prada, tells the March issue of Women's Health. "My shoulders puffed out, my back looked like there was a barrel of snakes in it and I had an eight-pack!"
That's not to say she didn't have a blast filming with Matt Damon (the duo share the cover of both Women's Health and Men's Health this month).
"I met Matt and he was lovely and very funny, and instantly, I thought, 'Oh we could be like teenage boys together,'" Blunt, who wed John Krasinski last July, tells Men's Health. "Just be weird and stupid and silly."
Damon, 40, adds that he has a similar relationship with longtime pal Ben Affleck.
"When one of us comes up with an absolutely f--king horrible idea, which happens all the time, the other person, without batting an eyelash, says, 'That's a terrible idea,'" he tells the mag. "And neither of us has our feelings hurt, and we just kind of move on."